Superoxide dismutase (SOD) enzymes are enzymes that catalyze the dismutation of the free radical superoxide, the one-electron reduction product of molecular oxygen. The dismutation of the free radical superoxide involves the conversion of this one-electron reduction product of molecular oxygen to the nonradical molecular oxygen. Superoxide dismutase enzymes are a class of oxidoreductases which contain either Cu/Zn, Fe, or Mn at the active site. Superoxide dismutase (SOD) mimetic compounds are low molecular weight catalysts which mimic the natural enzyme function of the superoxide dismutase enzymes. Thus, superoxide dismutase mimetic compounds also catalyze the conversion of superoxide into oxygen and hydrogen peroxide, rapidly eliminating the harmful biologically generated superoxide species that are believed to contribute to tissue pathology in a number of diseases and disorders. These diseases and disorders include reperfusion diseases, such as those following myocardial infarct or stroke, inflammatory disorders such as arthritis, and neurological disorders such as Parkinson's disease. Chem Reviews, 1999 vol 99, No. 9, 2573-2587.
Superoxide dismutase mimetic compounds possess several advantages over the superoxide dismutase enzymes themselves in that their chemical properties can be altered to enhance stability, activity and biodistribution while still possessing the ability to dismutase the harmful superoxide. Superoxide dismutase mimetic compounds have generated intense interest and have been the focus of considerable efforts to develop them as a therapeutic agent for the treatment of a wide range of diseases and disorders, including reperfusion injury, ischemic myocardium post-ischemic neuropathies, inflammation, organ transplantation and radiation induced injury. Most of the superoxide dismutase mimics currently being developed as therapeutic agents are synthetic low molecular weight manganese-based superoxide dismutase mimetic compounds. Chem Reviews, 2576.
Superoxide dismutase mimetic compounds are metal complexes in which the metal can coordinate axial ligands. Examples of such metal complexes include, but are not limited to, complexes of the metals Mn and Fe. Many of the complexes of the metals Mn and Fe do not possess superoxide dismutase activity but possess properties that enable them to be put to other therapeutic and diagnostic uses. These therapeutic and diagnostic uses include MRI imaging enhancement agents, peroxynitrite decomposition catalysts, and catalase mimics. These metal complexes, however, share the structural similarity of possessing a metal that can coordinate exchangeable ligands. These metal complexes exist in water as a mixture of species in which various ligands are possible. An illustration of such a mixture is provided by M40403, a Mn(II) complex of a nitrogen-containing fifteen membered macrocyclic ligand, shown in Scheme 1. One of the forms for this metal complex is the dichloro complex, which when dissolved in water another form is generated where one of the chloride anions immediately dissociates from the metal generating the [Mn(Cl)(aquo)]+ complex. The problem in aqueous solvent systems or any solvent which has a potential donor atom is that there are a variety of potential ligands available to coordinate axially to the Mn(II) ion of the complex. In conducting an analysis of a sample containing a metal complex by high performance liquid chromatography (HPLC) the chromatogram tends to be very broad and unresolved due to the presence of the various species of complexes, as shown in Scheme 1. This phenomena makes the identification and quantification of metal complexes by standard HPLC techniques quite difficult. Therefore, in light of the developing roles of metal complexes as therapeutics in the treatment of various disorders and diagnostic agents, a substantial need exists for an effective and workable high performance liquid chromatography method for analyzing metal complexes.

An additional complication which exists is the issue of the acid stability of the metal complex. As the pH decreases, the rate at which the complex becomes protonated and experiences instability increases. This presents particular problems for the use of HPLC as a method of detection and quantification of the metal complexes because the mobile phase used for reverse phase HPLC frequently contains mixtures of organic solvents and water in various combinations with trifluoroacetic acid. The trifluoroacetic acid is commonly present between about 0.1 to about 0.5% by weight. The presence of the trifluoroacetic acid causes the complex to dissociate. This dissociation destroys the potential of any such method to be used for release testing for purity. Furthermore, the trifluoroacetate anion causes the formation of some of the trifluoroacetato complex which could possess a different retention time from the chloro complexes thus, confusing the chromatography. Thus, the phenomenon of ligand exchange, coupled with the acid instability of the metal complexes, provides considerable challenges to the effort to detect and quantify metal complexes using HPLC. These challenges and needs have surprisingly been met by the invention described below.
Analytical HPLC is a powerful method to obtain information about a sample compound including information regarding identification, quantification and resolution of a compound. HPLC has been used particularly for the analysis of larger compounds and for the analysis of inorganic ions for which liquid chromatography is unsuitable. Skoog, D. A., West, M. A., Analytical Chemistry, 1986, p. 520. As an analytical tool HPLC takes advantage of the differences in affinity that a particular compound of interest has for the stationary phase and the mobile phase (the solvent being continuously applied to the column). Those compounds having stronger interactions with the mobile phase than with the stationary phase will elute from the column faster and thus have a shorter retention time. The mobile phase can be altered in order to manipulate the interactions of the target compound and the stationary phase. In normal-phase HPLC the stationary phase is polar, such as silica, and the mobile phase is a nonpolar solvent such as hexane or isopropyl ether. In reversed-phase HPLC the stationary phase is non-polar, often a hydrocarbon, and the mobile phase is a relatively polar solvent. Since 1974 when reversed-phase packing materials became commercially available, the number of applications for reversed-phase HPLC has grown, and reversed-phase HPLC is now the most widely used type of HPLC. Reversed-phase HPLC's popularity can be attributed to its ability to separate a wide variety of organic compounds. Reversed-phase chromatography is especially useful in separating the related components of reaction mixtures, and therefore is a useful analytical tool for determining the various compounds produced by reactions.
To create a non-polar stationary phase silica or synthetic polymer based adsorbents are modified with hydrocarbons. The most popular bonded phases are C1, C4, C8 and C18. Silica based adsorbents modified with trimethylchlorosilane (C1) and butyldimethylchlorosilane (C4) have a few applications in HPLC, mainly for protein separation or purification. These adsorbents show significant polar interactions. Octyl (C8) and octadecyl (C18) modified adsorbents are the most widely used silica based adsorbents, with almost 80% of all HPLC separations being developed with these adsorbents.
The most important parameter in reversed-phase HPLC is the mobile phase. The type of mobile phase employed in the HPLC will have a significant effect on the retention of the analytes in the sample, and varying the composition of the mobile phase allows the chromatographer to adjust the retention times of target components in the mixture to desired values. This ability provides the HPLC method with flexibility. The mobile phase in reversed-phase chromatography has to be polar and it also has to provide reasonable competition for the adsorption sites for the analyte molecules. Solvents that are commonly employed as eluent components in reversed-phase HPLC are acetonitrile, dioxane, ethanol, methanol, isopropanol, tetrahydrofuran, and water. In reversed phase HPLC of high molecular weight biological compounds, the solvents acetonitrile, isopropanol or propanol are most frequently used. Popular additives to the mobile phase for the improvement of resolution include mixtures of phosphoric acid and amines and perfluorinated carboxylic acids, especially trifluoroacetic acid (TFA).
HPLC exploits the differences in affinity that a particular compound of interest has for the stationary phase and the mobile phase. This phenomenon can be utilized to separate compounds based on the differences in their physical properties. Thus, HPLC can be used to separate stereoisomers, diastereomers, enantiomers, mirror image stereoisomers, and impurities. Stereoisomers are those molecules which differ from each other only in the way their atoms are oriented in space. The particular arrangement of atoms that characterize a particular stereoisomer is known as its optical configuration, specified by known sequencing rules as, for example, either + or − (also D or L) and/or R or S. Stereoisomers are generally classified as two types, enantiomers or diastereomers. Enantiomers are stereoisomers which are mirror-images of each other. Enantiomers can be further classified as mirror-image stereoisomers that cannot be superimposed on each other and mirror-image stereoisomers that can be superimposed on each other. Mirror-image stereoisomers that can be superimposed on each other are known as meso compounds. Diastereomers are stereoisomers that are not mirror images of each other. Diastereomers have different physical properties such as melting points, boiling points, solubilities in a given solvent, densities, refractive indices, etc. Diastereomers can usually be readily separated from each other by conventional methods, such as fractional distillation, fractional crystallization, or chromatography, including HPLC.
Enantiomers, however, present special challenges because their physical properties are identical. They generally cannot be separated by conventional methods, especially if they are in the form of a racemic mixture. Thus, they cannot be separated by fractional distillation because their boiling points are identical and they cannot be separated by fractional crystallization because their solubilites are identical (unless the solvent is optically active). They also cannot be separated by conventional chromatography such as HPLC because (unless the adsorbent is optically active) they are held equally onto the adsorbent. HPLC methods employing chiral stationary phases are a very common approach to the separation of enantiomers. To be able to separate racemic mixtures of stereoisomers, the chiral phase has to form a diastereomeric complex with one of the isomers, or has to have some other type of stereospecific interaction. The exact mechanism of chiral recognition is not yet completely understood. In reversed-phase HPLC a common type of chiral bonded phase is chiral cavity phases.
The ability to be able to separate diastereomers and enantiomers by HPLC is a useful ability in evaluating the success of synthetic schemes. It is often desirable to separate stereoisomers as a means of evaluating the enantiomeric purity of production samples. All references listed herein are hereby incorporated by reference in their entirety.